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52 terms

Principles of radiation physics

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Which of the following modes of a trifield image intensifier will result in the highest patient dose?
Its 12-inch mode
Most image intensifier tubes are either
dual-field or trifield, indicating the diameter of the input phosphor.
When a change to a smaller diameter mode is made,
the voltage on the electrostatic focusing lenses is increased, and the result is a magnified, but dimmer, image
The milliamperage will automatically be increased to compensate for the loss in brightness with a magnified image, resulting in
higher patient dose in the smaller-diameter modes.
A high-speed electron entering the tungsten target is attracted to the positive nucleus of a tungsten atom and, in the process, is decelerated. This results in
Bremsstrahlung radiation
If the positive nucleus of a tungsten atom attracts the electron, changing its course, a certain amount of energy is released during the
"braking" action
This energy is given up in the form of an x-ray photon called
Bremsstrahlung ("braking") radiation
Characteristic radiation
also produced at the target (less frequently) when an incident electron ejects a K-shell electron, and an L-shell electron drops into its place. Energy is liberated in the form of a characteristic ray, and its energy is representative of the difference in energy levels.
To be used more efficiently by the x-ray tube, alternating current is changed to unidirectional current by the
rectifiers
The rectification system is located
between the secondary coil of the high-voltage transformer and the x-ray tube
The filament transformer
functions to adjust the voltage and current going to heat the x-ray tube filament.
The autotransformer
varies the amount of voltage being sent to the primary coil of the high-voltage transformer so that the appropriate kVp can be obtained.
The high-voltage transformer
"steps up" the voltage to the required kilovoltage and steps down the amperage to milliamperage.
Patient dose during fluoroscopy is affected by the
1. distance between the patient and the input phosphor.
2. amount of magnification.
3. tissue density.
Moving the image intensifier closer to the patient during fluoroscopy
decreases the SID and patient dose (as SID is reduced, the intensity of the x-ray photons at the image intensifier's input phosphor increases; the automatic brightness control then automatically decreases the mA, and therefore patient dose).
Moving the image intensifier closer to the patient during fluoroscopy also decreases
the OID, and therefore magnification.
The source of electrons within the x-ray tube is
thermionic emission
The x-ray tube filament is made of thoriated tungsten and is part of the
cathode assembly
Which of the following terms describes the amount of electric charge flowing per second?
Current
Current
defined as the amount of electric charge flowing per second
Voltage
the potential difference existing between two points
Resistance
the property of a circuit that opposes current flow
Capacitance
describes a quantity of stored electricity
Which of the following occurs during Bremsstrahlung radiation production?
An electron approaching a positive nuclear charge changes direction and loses energy
The total number of x-ray photons produced at the target is contingent on the
1. tube current.
2. target material.
3. square of the kilovoltage.
The higher the atomic number
the denser and more closely packed the atoms making up the material are, and therefore the greater the chance of an interaction between a high-speed electron and the target material.
Tungsten alloy is the usual choice for the anode target material of radiographic equipment because it
1. has a high atomic number.
2. has a high melting point.
3. can readily dissipate heat.
With three-phase equipment, the voltage across the x-ray tube
1. is 87% to 96% of the maximum value.
2. is at nearly constant potential.
With single-phase, full-wave-rectified equipment, the voltage is constantly changing from
0% to 100% of its maximum value, It drops to 0 every 180° (of the AC waveform); that is, there is 100% voltage ripple.
Three-phase, six-pulse equipment has
a 13% voltage ripple
three-phase, 12-pulse equipment has
a 3.5% ripple. Therefore, the voltage never falls below 87% to 96.5% of its maximum value with three-phase equipment, and it closely approaches constant potential [direct current (DC)].
Off-focus, or extrafocal, radiation may be minimized by
restricting the x-ray beam as close to its source as possible
Off-focus, or extrafocal, radiation is produced as
electrons strike metal surfaces other than the focal track and produce x-rays that emerge with the primary beam at a variety of angles
This radiation is responsible for
indistinct images outside the collimated field.
Mounting a pair of shutters as close to the source as possible
minimizes off-focus radiation.
Which of the following devices is used to control voltage by varying resistance?
Rheostat
The autotransformer
operates on the principle of self-induction and functions to select the correct voltage to be sent to the high-voltage transformer to be "stepped up" to kilovoltage
The high-voltage transformer
increases the voltage and decreases the current.
The rheostat
is a type of variable resistor that is used to change voltage or current values. It is frequently found in the filament circuit
A fuse
is a device used to protect the circuit elements from overload by opening the circuit in the event of a power surge.
Determining mR/mAs output is often done to
determine linearity among x-ray machines
all the equipment being compared must be
of the same type (eg, all single-phase; all three-phase, six-pulse; etc)
If there is linearity among these machines, then
identical technique charts can be used
The vast majority of target interactions involve
the incident electrons and outer-shell tungsten electrons. No ionization occurs, and the energy loss is reflected in heat generation.
This presents a serious heat buildup problem in the anode, as heat production is directly proportional to
tube current.
An automatic exposure control device can operate on which of the following principles?
1. A photomultiplier tube charged by a fluorescent screen
2. A parallel-plate ionization chamber charged by x-ray photons
A phototimer is
one type of AEC that actually measures light. As x-ray photons penetrate and emerge from a part, a fluorescent screen beneath the image receptor glows, and the fluorescent light charges a photomultiplier tube. Once a predetermined charge has been reached, the exposure automatically terminates.
A parallel-plate ionization chamber
A radiolucent chamber is located beneath the patient (between the patient and the film). As photons emerge from the patient, they enter the chamber and ionize the air within it. Once a predetermined charge has been reached, the exposure is automatically terminated.
Motion of magnetic fields inducing current in a conductor refers to
the principle of mutual induction.
Which of the following is (are) characteristics of the x-ray tube?
1. The target material should have a high atomic number and a high melting point.
2. The useful beam emerges from the port window.
3. The cathode assembly receives both low and high voltages.
The cathode filament receives
low-voltage current to heat it to the point of thermionic emission. Then high voltage is applied to drive the electrons across to the focal track.
Which of the following will serve to increase the effective energy of the x-ray beam?
1. Increase in added filtration
2. Increase in kilovoltage